Sensor

ABSTRACT

For a sensor whose sensor structure is implemented in a micromechanical structural component and which has parts which are movable in relation to the stationary substrate of the structural component, and which also includes  
     an unsupported seismic mass ( 1 ),  
     a spring system having at least one spring ( 2 ), the seismic mass ( 1 ) being connected to the substrate through the spring system,  
     an overload protection to limit the deflection of the spring system and the seismic mass ( 1 ) in at least one direction, and  
     means for detecting the deflections of the spring system and the seismic mass ( 1 ), design measures are proposed whereby the impact forces may be reduced to prevent conchoidal breaks and resulting incipient damage to the sensor structure, as well as formation of particles. To that end, at least one two-dimensional stop ( 3 ) for at least one moving part of the sensor structure is provided as overload protection. Alternatively or in addition thereto, at least one spring stop ( 7 ) for at least one moving part of the sensor structure is provided according to the present invention as overload protection.

BACKGROUND INFORMATION

[0001] The present invention relates to a sensor whose sensor structureis implemented in a micromechanical structural component and has partsthat are movable in relation to the stationary substrate of thestructural component. The sensor structure includes at least oneunsupported seismic mass and a spring system having at least one spring,the seismic mass being connected with the substrate through the springsystem. The sensor structure also includes an overload protection deviceto limit the deflection of the spring system and the seismic mass in atleast one direction. The sensor is also equipped with means fordetecting the deflections of the spring system and the seismic mass.

[0002] A sensor of this type, which is designed as an accelerationsensor is known from practice. The seismic mass of that sensor isconstructed in the form of a rocker which is both mechanically andelectrically connected to the stationary substrate of the structuralcomponent. While the distribution of mass of the rocker is asymmetricalin relation to the torsion spring system, the rocker has two capacitorsurfaces which are arranged and constructed symmetrically in relation tothis spring system, and each of which forms a capacitor together withthe substrate. An acceleration acting on the sensor structure causes therocker to rotate or tilt around the spring system, thus causing a changein the difference of capacitances between these two capacitors. Byevaluating the difference of capacitances or its change, it is possibleto determine the acceleration acting on the sensor structure. The knownacceleration sensor has a vertical sensitivity, so that accelerationsperpendicular to the plane of the chip are detectable with the knownsensor.

[0003] The moving parts of the sensor structure of the known sensor mayonly be deflected within certain limits without mechanical damage suchas a broken spring or electrical short circuits occurring. Overloadaccelerations may also result in greater deflections of the movingparts, however, and thus to corresponding damage. For that reason, theknown acceleration sensor is equipped with stops for the torsionsprings. These stops are located in the area of connection between thetorsion spring and the rocker and are rigidly connected to the substrateof the structural component, so that the deflection of the torsionsprings and the motion of the rocker in the x/y plane, i.e. parallel tothe plane of the chip, is limited. The geometry of the stops is notadapted to the expected deformation and deflection of the torsionsprings, so that the stops and the torsion springs make contact only atpoints or edges in the event of a corresponding overload acceleration.

[0004] In regard to acceleration in the z direction two cases must bedistinguished: acceleration into the substrate and acceleration out ofthe substrate. In the first of these cases the motion of the torsionspring and of the seismic mass is easily limited by a stop at anelectrically neutral location on the substrate. In contrast, a motion ofthe seismic mass out of the substrate, as is to be expected in thesecond case, is not easily limited. Hence in the case of a structuralheight of around 10 μm, deflections of 10 μm or more may result in theseismic mass being elevated out of its surroundings, and consequently inthe sensor structure getting stuck.

[0005] In non-directed drop tests of the known acceleration sensor,conchoidal breaks were found on the outer edge of the stops and on thetorsion spring. Such conchoidal breaks may modify the mechanicalproperties of the spring system or result in development of cracks asincipient damage to the sensor structure. This may cause changes to thecharacteristics of the sensor such as sensitivity, offset and testsignal. Furthermore, conchoidal breaks are a source of particles thatmay cause electrical short circuits or also mechanical blocking of therocker. On the whole, the aforementioned conchoidal breaks usuallyresult in quality-relevant degradation of the sensor function, and inextreme cases may even result in total failure of the sensor function.

ADVANTAGES OF THE INVENTION

[0006] The present invention proposes two design measures that allow theimpact forces in a sensor of the type named at the beginning to bereduced, in order to prevent conchoidal breaks and related incipientdamage to the sensor structure as well as particle formation.

[0007] This is achieved according to the present invention in part bythe fact that a stop for a moving part of the sensor structure, whichfunctions as an overload protection device, is of a flat design, so thatin the event of a corresponding overload acceleration contact betweenthe moving part and this stop occurs via a flat surface. This enablesthe impact forces to be distributed to the sensor structure and absorbedby it more uniformly.

[0008] In addition, it is proposed according to the present inventionthat a stop for a moving part of the sensor structure, which functionsas a means of overload protection, be of an elastic design. In this casethe impact forces are reduced by having the kinetic energy converted atleast partially into flexural energy. The conversion of the kineticenergy into flexural energy may be influenced by the design of thespring stop.

[0009] It should be pointed out here that the two measures explainedabove for reducing the impact forces may be implemented independently ofeach other or may also be combined. The present invention thereforeincludes both sensor structures having two-dimensional stops and sensorstructures having spring stops, and also sensor structures that includeboth two-dimensional stops and spring stops, as well as sensorstructures with stops that are both two-dimensional and of elasticdesign.

[0010] In principle, there are various possibilities for implementingand arranging a two-dimensional stop in the sensor structure of a sensoraccording to the present invention.

[0011] In view of easy implementation in the sensor structure, it isadvantageous if the spring stop includes at least one spiral springwhich is connected to the substrate at one end. The length and width ofthe spiral spring are available here as free design parameters by whichthe conversion of the kinetic energy into flexural energy may beinfluenced. A particularly smooth, gradual deceleration of the movingparts of the sensor structure is attainable in an advantageous way withthe help of a multi-stage spring stop that includes a plurality ofspiral springs arranged essentially parallel to each other. The spiralsprings of such a multi-stage spring stop may vary in length and/orwidth, depending on what springing effect is to be achieved. The elasticeffect also depends here on the intervals between the individual spiralsprings-that is, on how far one spiral spring of the multi-stage stopmay be deflected without touching the adjacent spiral springs of thestop. Independent of the length and width of the individual spiralsprings, this spacing is predefinable in an advantageous way, using nubswhich are advantageously formed on the individual spiral springs in thearea of the free ends.

[0012] In an advantageous variant of the sensor according to the presentinvention, at least one spring stop is provided for at least one springof the spring system. In this connection, it is advantageous if the atleast one spiral spring of the spring stop is positioned essentiallyparallel to the spring, so that the end of the spring connected to theseismic mass and the free end of the spiral spring point in the samedirection. With such an arrangement of spring and spiral spring, thespiral spring is able to follow the motion of the spring and thus todecelerate the motion of the spring in a particularly smooth manner.

[0013] In addition or alternatively, at least one spring stop may beprovided for the seismic mass. In a first variant of the sensor, thespring stop might simply be positioned essentially parallel to one sideof the seismic mass. However, it is advantageous for reasons of space ifthe seismic mass has at least one cutout, and the at least one spiralspring of the spring stop is positioned essentially parallel to at leastone side wall of this cutout. A particularly good deceleration effect isachievable if the cutout is in the edge area of the seismic mass, andthe at least one spiral spring of the spring stop is positioned so thatat least its free end projects into the cutout.

[0014] Two-dimensional and spring stops like those explained above maybe integrated in an advantageous manner into the micromechanical sensorstructure of a sensor according to the present invention if these stopsare intended to limit the deflection of the spring system and of theseismic mass only in the x/y direction-that is, deflections in a planeoriented parallel to the main plane of the structural component.Chipping and associated formation of particles may additionally beprevented in this case by having at least part of the comer regions inthe sensor structure provided with roundings or rounded off.

DRAWING

[0015] As explained in detail above, there are various possibilities ofadvantageously designing and refining the teaching of the presentinvention. We refer to the claims subordinate to claims 1 and 5, as wellas to the following explanation of a number of exemplary embodiments onthe basis of the drawing.

[0016]FIG. 1 shows a detail of a top view of a sensor structure with atwo-dimensional stop of a first sensor according to the presentinvention.

[0017]FIG. 2 shows a detail of a top view of a sensor structure with aspring stop of a different sensor according to the present invention.

[0018]FIG. 3 shows various stop positions on the seismic mass of asensor according to the present invention.

[0019]FIG. 4 shows a detail of a top view of a sensor structure with aspring stop of a different sensor according to the present invention.

[0020]FIG. 5 shows additional possible stop positions on the seismicmass of a sensor according to the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0021] Each of the sensor structures depicted in FIGS. 1 through 5 isimplemented in a micromechanical structural component, and includesparts that are movable in relation to the stationary substrate of thestructural component, namely an unsupported seismic mass 1 and a springsystem having at least one spring 2. Seismic mass 1 is connected to thesubstrate through a spring system, so that the distribution of mass ofseismic mass 1 is asymmetrical in reference to the spring system. All ofthe sensor structures depicted in FIGS. 1 through 5 are designed for usein an acceleration sensor having horizontal and vertical sensitivity, inthat unsupported seismic mass 1 is designed in the form of a rocker andthe spring system includes at least one torsion spring 2. Accelerationsacting on the sensor structure are detected and determined in this casethrough the corresponding deflections of the spring system and of theseismic mass.

[0022] In addition, in each of the sensor structures depicted in FIGS. 1through 5, a means for overload protection is provided to limit thedeflection of the spring system and the seismic mass in at least onedirection.

[0023] The sensor structure depicted in FIG. 1 has a two-dimensionalstop 3 as overload protection for torsion spring 2. This two-dimensionalstop 3 is implemented in the form of a solid structure placed on thesubstrate and limits the deflection of torsion spring 2 in the x/ydirection that is, in a plane that is oriented parallel to the mainplane of the structural component. To prevent edge contact of torsionspring 2 and ensure surface contact, stop 3 is constructed as a slopewhose inclination is derived from the bending line of torsion spring 2.Alternatively, the two-dimensional stop could also have a curvature, forexample the curvature of a hyperbola.

[0024] Torsion spring 2 of the sensor structure depicted in FIG. 1 has arounding 5, 6, both in the region of transition to seismic mass 1 and inthe region of transition to “mainland” 4, i.e. to the region in whichtorsion spring 2 is connected to the substrate of the structuralcomponent. The function of these roundings is to reduce the tension invertical deflections of torsion spring 2, i.e. in deflectionsperpendicular to the main plane of the structural component. That makesit possible to reduce the probability of a break in drop tests.

[0025] In connection with FIG. I it should also be noted that within thescope of the invention a two-dimensional stop may also be provided asoverload protection for other moving parts of the sensor structure,namely for the seismic mass for example. The various possibilities fordesigning such a stop in the sensor structure are explained in greaterdetail in conjunction with FIGS. 3 and 5.

[0026]FIG. 2 depicts a multi-stage spring stop 7 for torsion spring 2,which limits the deflection of torsion spring 2 in the x/y directionjust as the two-dimensional stop shown in FIG. 1 does. To that end, thespring stop includes a total of four spiral springs 8, 9 and 10, 11,which are positioned on both sides of torsion spring 2 essentiallyparallel to it. Spiral springs 8, 9 and 10, 11 are all connected at oneend to the substrate of the structural component, so that their freeends and the end of torsion spring that is connected to seismic mass 1all point in the same direction. The individual spiral springs 8, 9 and10, 11 of the spring stop depicted here differ in length. Inner spiralsprings 8 and 10, immediately adjacent to torsion spring 2, aresignificantly longer than the two outer spiral springs 9 and 1 1. Thedeflection of the individual spiral springs 8, 9 and 10, 11 is limitedby nubs 12 which are formed in the region of the free ends of spiralsprings 8, 9 and 10, 11. In addition, the free ends of spiral springs 8,9 and 10, 11 are provided with roundings 14, to prevent chipping in thisarea.

[0027] Central channel 13 between the two inner spiral springs 8 and 10forms a guide for torsion spring 2 during vertical accelerations thatcause seismic mass 1 to be elevated out of the substrate level. In thesecases central channel 13 prevents seismic mass 1 from getting caught asit returns to its starting position. Therefore there should be no nubsprotruding into central channel 13. In addition, spiral springs 8, 9 and10, 11 should have no less than a critical bending strength, in order toprevent torsion spring 2 from adhering on spiral springs 8 and 10. Thecritical bending strength is usually determined by experiment, withexperience with comparable similar sensor structures able to provide astarting point.

[0028] In a particularly advantageous variant of the sensor structuredepicted in FIG. 2, the sides of spiral springs 8 and 10 facing torsionspring 2 are beveled, so that the contact against the spring stop alsois not on an edge but flat.

[0029] In the sensor structure of a sensor according to the presentinvention, in addition to or alternatively to the stops described abovewhich are used as an overload protection, there may also betwo-dimensional or spring stops provided for the seismic mass. FIG. 3shows various possibilities for positioning such stops. There areessentially three conceivable areas: the area directly on torsion spring2—stops 31; an area on the inner edge of seismic mass 1-stops 32; andthe outer area of seismic mass 1—stops 33.

[0030] Stops 33 located in the outer edge area of seismic mass 1 havethe advantage that during rotational accelerations the motion of seismicmass 1 ends in the stop even in the case of slight deflections. Thepulse transfer is correspondingly slight.

[0031] As mentioned earlier, acceleration perpendicular to the mainplane of the structural component may result in the seismic mass beingelevated out of the sensor structure and consequently the sensorstructure becoming caught, if the motion of the seismic mass in this zdirection is not limited. If no corresponding z stop is present, it isadvantageous to position the stops as close as possible to the axis ofrotation of the seismic mass, since the elevation of the seismic mass isminimal at this location in the sensor structure.

[0032]FIG. 4 depicts a possibility of implementing a spring stop onseismic mass 1. The seismic mass here has two cutouts 41 in the edgearea. The free ends of two spiral springs 42, which are positionedessentially parallel to the side walls of the two cutouts 41, projectinto these two cutouts 41. The free ends of spiral springs 42 andcutouts 41 all have nubs, so that the resulting stop intervals betweenspiral springs 42 within a cutout 41 and the side walls of thecorresponding cutout 41 differ. That results in stepwise stopping. Inaddition, the bending strength of spiral springs 42, which is influencedby the respective lengths and widths of spiral springs 42, is chosen sothat the resulting restoring forces are sufficient to prevent spiralsprings 42 from adhering on seismic mass 1.

[0033] Finally, it should also be pointed out that the spring stopdescribed above may also be designed as a two-dimensional stop for theseismic mass, and may be combined with the roundings of torsion spring 2described in conjunction with FIG. 1.

[0034] In principle, it is also possible to position stops 51, 52 withinseismic mass 1, as illustrated in FIG. 5. Stops 51 are located atextensions of torsion spring 2 within the suspension mounts of thecapacitor surfaces, through which the deflections of the seismic massare determined here. This arrangement of stops 51 permits simpleelectrical contacting.

[0035] When positioning stops in a sensor structure, such as those thatare part of a sensor according to the present invention, the followingaspects must be kept in mind:

[0036] Stops which limit the motion of at least one spring of the springsystem in the x/y direction also act as guides for the spring if theseismic mass is elevated out of the x/y plane by vertical acceleration,so that the seismic mass may slide back into its starting positionagain. In this variant, overload accelerations in the z direction do notnecessarily result in a failure of the sensor function. In the event ofincipient damage, these stops may significantly influence the propertiesof the sensor, such as sensitivity, test signal, and offset, since thesprings are a mechanically very sensitive structure.

[0037] In contrast, stops which limit the motion of the seismic mass inthe x/y direction do not affect the mechanical properties and thefunctionality of the spring system. However, these stops also do notfunction as guides in deflections of the seismic mass in the zdirection, so that it is easier here for the seismic mass to get stuckand remain on the sensor structure, which results in a failure of thesensor function. In addition, incipient damage of the sensor structureby particle formation is not detectable in this case, because the sensorproperties or the characteristic parameters do not change. Faults in thesensor function caused by wandering particles are also not detectable.

What is claimed is:
 1. A sensor whose sensor structure is implemented ina micromechanical structural component and which has parts which aremovable in relation to the stationary substrate of the structuralcomponent, including at least an unsupported seismic mass (1), a springsystem having at least one spring (2), the seismic mass (1) beingconnected to the substrate through the spring system, an overloadprotection to limit the deflection of the spring system and the seismicmass (1) in at least one direction, and means for detecting thedeflections of the spring system and the seismic mass (1), wherein atleast one two-dimensional stop (3) for at least one moving part of thesensor structure is provided as overload protection.
 2. The sensor asrecited in claim 1, wherein at least one two-dimensional stop (3) for atleast one spring (2) of the spring system is provided.
 3. The sensor asrecited in claim 2, wherein the two-dimensional stop (3) is tilted orcurved in the direction of the deflection of the spring (2).
 4. Thesensor as recited in one of claims 2 or 3, wherein the geometry of thetwo-dimensional stop (3) is adapted to the bending line of the spring(2).
 5. A sensor whose sensor structure is implemented in amicromechanical structural component and which has parts which aremovable in relation to the stationary substrate of the structuralcomponent, including at least an unsupported seismic mass (1), a springsystem having at least one spring (2), the seismic mass (1) beingconnected to the substrate through the spring system, an overloadprotection to limit the deflection of the spring system and the seismicmass (1) in at least one direction, and means for detecting thedeflections of the spring system and the seismic mass (1), wherein atleast one spring stop (7) for at least one moving part of the sensorstructure is provided as overload protection.
 6. The sensor as recitedin claim 5, wherein the spring stop (7) includes at least one spiralspring (8 through 11) connected to the substrate on one end.
 7. Thesensor as recited in claim 6, wherein the spring stop (7) is ofmulti-stage design, in that it includes a plurality of spiral springs (8through 11) positioned essentially parallel to each other.
 8. The sensoras recited in claim 7, wherein the spiral springs (8 through 11) of themulti-stage spring stop (7) differ in length and/or width.
 9. The sensoras recited in one of claims 5 through 8, wherein at least one nub (12)is formed on the at least one spiral spring (8 though 11), preferably inthe area of its free end, and at least one area of the surface of thenub forms a stop face of the spiral spring (8 through 11).
 10. Thesensor as recited in one of claims 5 through 9, wherein at least onespring stop (7) is provided for at least one spring (2) of the springsystem.
 11. The sensor as recited in one of claims 6 through 9 and claim10, wherein the at least one spiral spring (8 through 11) of the springstop (7) is positioned essentially parallel to the spring (2), so thatthe end of the spring (2) connected to the seismic mass (1) and the freeend of the spiral spring (8 through 11) point in the same direction. 12.The sensor as recited in one of claims 5 through 11, wherein at leastone spring stop is provided for the seismic mass (1).
 13. The sensor asrecited in one of claims 6 through 9 and claim 12, wherein the seismicmass (1)) has at least one cutout (41), and the at least one spiralspring (42) of the spring stop is positioned essentially parallel to atleast one side wall of the cutout (41).
 14. The sensor as recited inclaim 13, wherein the cutout (41) is located in the edge area of theseismic mass (1) and the at least one spiral spring (42) is positionedso that at least its free end projects into the cutout (41).
 15. Thesensor as recited in one of claims 1 through 4 and/or one of claims 5through 14, wherein the stop (3; 7) limits the deflection of the springsystem and the seismic mass (1) in the x/y direction, i.e. in a planethat is oriented parallel to the main plane of the structural component.16. The sensor as recited in claim 15, wherein at least one part of thecorner areas in the sensor structure is provided with roundings (5, 6;14) or is rounded off.
 17. An acceleration sensor having horizontal andvertical sensitivity as recited in one of claims 1 though 16, whereinthe unsupported seismic mass (1) is constructed in the form of a rockerand the spring system includes at least one torsion spring (2).